It is known that quantum well devices can bind electrons having energy less than the barrier heights of the constituent wells. Indeed such devices have been employed as infrared photodetectors. For example U.S. Pat. No. 4,894,526 issued Jan. 16, 1990, to Bethea et al., discloses a detector wherein infrared radiation incident on a superlattice of doped quantum wells gives rise to intersubband resonance radiation which excites electrons from the ground state in a well into an excited state. A photocurrent results from excited electrons tunneling out of the quantum wells. U.S. Pat. No. 5,023,685, issued Jun. 11, 1991, to Bethea et al., also shows a quantum well radiation detector. The wells have but a single bound energy state.
The present applicant has discovered that, in addition to having electron bound states within a quantum well at energies below the barrier height, devices can be fabricated which have localized electron states at energies above the barrier height. This discovery permits the fabrication of a variety of new detectors and modulators.